Capacitors are frequently employed in measuring circuits such as in an electrometer or a sample and hold circuit, as well as in other applications. Such measuring capacitors are typically charged by a charging circuit to some level representative of an input signal. For example, an electrometer may be employed for measuring radiation exposure from a radiation source such as an x-ray generator or radiation treatment machine. The accuracy of the measurement depends on determining the amount of charge which has been deposited on the capacitor. If the value of the capacitor and the voltage across it are known, then the charge is the product of the values of the capacitor and the voltage. The charge may be applied to the capacitor as by a sample and hold circuit, or another source such as a current source in an electrometer wherein the capacitor serves as an integrating capacitor. It is known that in such applications, an error takes place following a charging interval. This error is known as the dielectric absorption effect. The error is a variation in the voltage across the capacitor with time following a charging interval. The period of time that this takes place may range from several milliseconds to several minutes. The effect is apparent when a known amount of charge is deposited on a capacitor and the voltage across the capacitor is then observed. The initial value of the voltage will decay slightly at an apparent exponential rate to a more stable, lower value of voltage, and then remain stable. Some of the charge on the capacitor has soaked into the dielectric and is no longer apparent in the voltage reading. If, after allowing the voltage to stabilize at this lower level, the capacitor is short-circuited, it would appear that all of the charge would be removed. However, if the short circuit is momentary only, then the voltage across the capacitor will begin at zero and then rise at a seemingly exponential rate to a low level which is similar to the amount of voltage drop experienced due to the initial charge soaking into the dielectric. The charge which has soaked into the dielectric has reappeared after the capacitor has discharged. Typical values of dielectric absorption for capacitors used in electrometers may be on the order of 0.03% to 1% of the total charge on the capacitor. This represents an error voltage that must be considered when employing a capacitor in a measuring application.
The error voltage caused by the dielectric absorption effect could be ignored providing the error voltage is tolerable for the application under consideration. Thus, if the error voltage represents an error on the order of 0.48%, this may be considered small enough for many applications.
Another solution to the problem is to employ a dielectric material for the capacitor that has better dielectric absorption characteristics. Thus, the dielectric absorption of a material is related to the molecular structure of the material. A polystyrene dielectric capacitor may have a relatively low dielectric absorption effect, such as a level of around 0.03%. However, it is also known that polystyrene dielectric capacitors are not stable and their characteristics may drift by, for example, 0.5% over a one-year period. On the other hand, a glass dielectric capacitor may be relatively stable such as on the order of 0.02% over the same period of time, but exhibit a much higher dielectric absorption effect such as on the order of 0.48%.
An air dielectric capacitor may also be employed and this exhibits a very small dielectric absorption effect. However, air dielectric capacitors are large and expensive. For example, an air dielectric capacitor may take up a volume on the order of 16 cubic inches for a 1,000 pico-farad capacitor, whereas a typical glass dielectric capacitor may have a size on the order of 0.1 cubic inch for a capacitor having a capacitance on the order of 1,000 pico-farads. Because of the size, an air capacitor is impractical for use with a portable electrometer, for example.
One way to minimize the effect of dielectric absorption in a capacitor is to wait a substantial period of time after the charging interval has terminated before taking a reading. Thus, by waiting for 60 seconds or more after the charging internal has terminated, the dielectric absorption effect error may be reduced from 0.48% (with a glass dielectric) to 0.01%. An electrometer employing a capacitor in this matter must be calibrated to allow for this error and this may not be reasonable in practice, particularly where a fast reset time on the order of 0.33 seconds is desired.
The U.S. Pat. No. 4,023,097 to R. Hanashey discloses means for compensating for dielectric absorption of a capacitor in a test instrument. In this application, an additional wire is placed adjacent to a measurement wire in the same shielded cable assembly. Since only one wire is used for the measurements, and both wires are subjected to substantially the same interfering signals (including dielectric absorption), the measured signal can be compensated by subtracting the signal obtained on the added wire from the signal obtained on the measurement wire.
The United States Patent to J. Reinertson et. al. 4,213,348 discloses a sample and hold circuit wherein a hold period is maintained a sufficient length of time to reduce to negligible levels the effects of the dielectric absorption of an associated capacitor. Other U.S. patents of interest relative to dielectric absorption effect compensation include the U.S. Pat. Nos. to Takatsuka 5,144,307 and J. Lerma 4,211,981.
A technical article entitled "A Wide-Range Logarithmic Charge Digitizer" by A. Richard Zacher appearing in IEEE Transactions on Circuits and Systems, Volume 40, No. 5, May, 1993 is also of interest. This article describes compensation for dielectric absorption in an integrating capacitor wherein a compensation cycle is provided before every measurement cycle. This precharges the integration capacitor to a fixed level which forces a consistent charge into the dielectric. This type of compensation adds a significant amount of time to a measuring operation.